| Doc. no. | P2315R0 |
| Date: | 2021-02-12 |
| Audience: | WG21 |
| Reply to: | Jonathan Wakely <lwgchair@gmail.com> |
Section: 23.5.3 [move.iterators], 23.5.6 [iterators.counted] Status: Tentatively Ready Submitter: Patrick Palka Opened: 2020-02-07 Last modified: 2020-02-10
Priority: 2
View all other issues in [move.iterators].
Discussion:
It is not possible to use the const & overloads of counted_iterator::base() or move_iterator::base() to get at an underlying move-only iterator in order to compare it against a sentinel.
More concretely, assuming issue LWG 3389 is fixed, this means thatauto v = r | views::take(5); ranges::begin(v) == ranges::end(v);
is invalid when r is a view whose begin() is a move-only input iterator. The code is invalid because ranges::take_view::sentinel::operator==() must call counted_iterator::base() to compare the underlying iterator against its sentinel, and therefore this operator==() requires that the underlying iterator is copy_constructible.
Suggested resolution: Make these const & base() overloads return the underlying iterator by const reference. Remove the copy_constructible constraint on these overloads. Perhaps the base() overloads for the iterator wrappers in 24.7 [range.adaptors] could use the same treatment?[2020-02 Prioritized as P2 Monday morning in Prague]
[2021-01-28; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4849.
Modify 23.5.3.2 [move.iterator], class template move_iterator synopsis, as indicated:
namespace std {
template<class Iterator>
class move_iterator {
public:
using iterator_type = Iterator;
[…]
constexpr const iterator_type& base() const &;
constexpr iterator_type base() &&;
[…]
};
}
Modify 23.5.3.5 [move.iter.op.conv] as indicated:
constexpr const Iterator& base() const &;
-1- Constraints: Iterator satisfies copy_constructible.-2- Preconditions: Iterator models copy_constructible.-3- Returns: current.
Modify 23.5.6.1 [counted.iterator], class template counted_iterator synopsis, as indicated:
namespace std {
template<input_or_output_iterator I>
class counted_iterator {
public:
using iterator_type = I;
[…]
constexpr const I& base() const & requires copy_constructible<I>;
constexpr I base() &&;
[…]
};
}
Modify 23.5.6.3 [counted.iter.access] as indicated:
constexpr const I& base() const &requires copy_constructible<I>;-1- Effects: Equivalent to: return current;
Section: 24.5.4.3 [range.subrange.access] Status: Tentatively Ready Submitter: Casey Carter Opened: 2020-04-21 Last modified: 2020-09-06
Priority: 2
Discussion:
subrange::advance(n) is specified to call ranges::advance(begin_, n, end_) in both StoreSize and !StoreSize cases (24.5.4.3 [range.subrange.access]/9). Unfortunately, ranges::advance(begin_, n, end_) has undefined behavior when n < 0 unless [end_, begin_) denotes a valid range (23.4.4.2 [range.iter.op.advance]/5). This would all be perfectly fine — the UB is exposed to the caller via effects-equivalent-to — were it not the design intent that subrange::advance(-n) be usable to reverse the effects of subrange::advance(n) when the subrange has a bidirectional iterator type. That is, however, clearly the design intent: subrange::prev(), for example, is equivalent to subrange::advance(-1).
[2020-05-11; Reflector prioritization]
Set priority to 2 after reflector discussions.
[2021-01-15; Issue processing telecon]
Set status to Tentatively Ready after discussion and poll.
| F | A | N |
|---|---|---|
| 7 | 0 | 3 |
Proposed resolution:
This wording is relative to N4861.
Modify 24.5.4.3 [range.subrange.access] as indicated:
constexpr subrange& advance(iter_difference_t<I> n);-9- Effects: Equivalent to:
(9.1) — If StoreSize is true,auto d = n - ranges::advance(begin_, n, end_); if (d >= 0) size_ -= to-unsigned-like(d); else size_ += to-unsigned-like(-d); return *this;
(9.2) — Otherwise,ranges::advance(begin_, n, end_); return *this;if constexpr (bidirectional_iterator<I>) { if (n < 0) { ranges::advance(begin_, n); if constexpr (StoreSize) size_ += to-unsigned-like(-n); return *this; } } auto d = n - ranges::advance(begin_, n, end_); if constexpr (StoreSize) size_ -= to-unsigned-like(d); return *this;
Section: 24.7.10.2 [range.drop.while.view] Status: Tentatively Ready Submitter: Michael Schellenberger Costa Opened: 2020-10-13 Last modified: 2020-11-07
Priority: 0
View all other issues in [range.drop.while.view].
Discussion:
Similar to ranges::filter_view 24.7.5.2 [range.filter.view] p3, ranges::drop_while_view should have a precondition on its begin() method that the predicate is set.
I propose to add as 24.7.10.2 [range.drop.while.view] p3:Preconditions: pred_.has_value().
[2020-11-07; Reflector prioritization]
Set priority to 0 and status to Tentatively Ready after six votes in favour during reflector discussions.
Proposed resolution:
This wording is relative to N4868.
Modify 24.7.10.2 [range.drop.while.view] as indicated:
Since we usually don't rely on implicit bool conversion in Preconditions: elements an explicit "is true" has been added. Editorial fixes of the referenced paragraph 24.7.10.2 [range.drop.while.view] p3 and similar places have been requested separately.
constexpr auto begin();-?- Preconditions: pred_.has_value() is true.
-3- Returns: ranges::find_if_not(base_, cref(*pred_)). -4- […]
Section: 24.7.16.3 [range.elements.iterator] Status: Tentatively Ready Submitter: Michael Schellenberger Costa Opened: 2020-10-28 Last modified: 2020-11-15
Priority: 0
View other active issues in [range.elements.iterator].
View all other issues in [range.elements.iterator].
Discussion:
During code review of elements_view for MSVC-STL we found two issues that should be easily addressed:
elements_view::iterator constraints both operator++(int) member functions
constexpr void operator++(int) requires (!forward_range<Base>); constexpr iterator operator++(int) requires forward_range<Base>;
However, given that a constrained method would be preferred we only need to constrain one of those. The proposal would be to remove the constraint from the void returning overload and change the declaration to
constexpr void operator++(int); constexpr iterator operator++(int) requires forward_range<Base>;
elements_view::iterator operator- is constrained as follows:
friend constexpr difference_type operator-(const iterator& x, const iterator& y) requires random_access_range<Base>;
However, that requires its base to have operator- defined. We should change the constraint to sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>:
friend constexpr difference_type operator-(const iterator& x, const iterator& y) requires sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>;
[2020-11-01; Daniel comments]
Bullet (2) of the discussion has already been resolved by LWG 3483, it has therefore been omitted from the proposed wording below.
[2020-11-15; Reflector prioritization]
Set priority to 0 and status to Tentatively Ready after five votes in favour during reflector discussions.
Proposed resolution:
This wording is relative to N4868.
This wording intentionally only touches operator++(int) and not operator-, see the 2020-11-01 comment for the reason why.
Modify 24.7.16.3 [range.elements.iterator], class template elements_view::iterator synopsis, as indicated:
[…][…] constexpr iterator& operator++(); constexpr void operator++(int)requires (!forward_range<Base>); constexpr iterator operator++(int) requires forward_range<Base>; […]constexpr void operator++(int)requires (!forward_range<Base>);-6- Effects: Equivalent to: ++current_.
constexpr iterator operator++(int) requires forward_range<Base>;[…]
Section: 24.7.15 [range.reverse], 24.7.7 [range.take], 24.7.9 [range.drop], 24.7.10 [range.drop.while], 24.7.14 [range.common], 24.7.10 [range.drop.while], 24.7.16 [range.elements] Status: Tentatively Ready Submitter: Barry Revzin Opened: 2020-11-01 Last modified: 2021-01-15
Priority: Not Prioritized
Discussion:
Consider the following approach to trimming a std::string:
auto trim(std::string const& s) {
auto isalpha = [](unsigned char c){ return std::isalpha(c); };
auto b = ranges::find_if(s, isalpha);
auto e = ranges::find_if(s | views::reverse, isalpha).base();
return subrange(b, e);
}
This is a fairly nice and, importantly, safe way to implement trim. The iterators b and e returned from find_if will not dangle, since they point into the string s whose lifetime outlives the function. But the status quo in C++20 is that s | views::reverse is not a borrowed range (because reverse_view<V> is never a borrowed range for any V). As a result, find_if(s | views::reverse, isalpha) returns dangling rather than a real iterator.
Instead, you have to write it this way, introducing a new named variable for the reversed view:
auto trim(std::string const& s) {
auto isalpha = [](unsigned char c){ return std::isalpha(c); };
auto b = ranges::find_if(s, isalpha);
auto reversed = s | views::reverse;
auto e = ranges::find_if(reversed, isalpha).base();
return subrange(b, e);
}
But borrowed range can be a transitive property. s itself is a borrowed range (as all lvalue references are) so s | views::reverse could be made to be too, which would allow the first example above to work with really no downside. We know such an iterator would not dangle, we just need to teach the library this.
P2017R1 resolves this by making reverse_view<V> a borrowed range when V is a borrowed range (and likewise several other range adapters).Rationale:
Resolved by P2017R1.[2021-01-15; Telecon prioritization]
Set status to Tentatively Ready after five P0 votes in reflector discussion.
Proposed resolution:
Section: 17.6.5 [ptr.launder] Status: Tentatively Ready Submitter: Hubert Tong Opened: 2020-11-10 Last modified: 2021-02-08
Priority: 3
View all other issues in [ptr.launder].
Discussion:
The wording in 17.6.5 [ptr.launder] paragraph 4:
An invocation of this function may be used in a core constant expression whenever the value of its argument may be used in a core constant expression.
can be taken to mean that the invocation may be used only when the value of its argument can be used in place of the invocation itself.
That interpretation is not particularly obvious, but based on comments on the CWG reflector (see here), that is the interpretation that matches the design intent. Consider:
#include <new>
struct A { int x; int y; };
struct B { float x; int y; };
union U {
A a;
B b;
};
constexpr A foo() {
U u;
int* byp = &u.b.y;
static_assert(&u.b.y == static_cast<void*>(&u.a.y));
u.a.y = 42;
*std::launder(byp) = 13;
return u.a;
}
extern constexpr A globA = foo();
If the static_assert succeeds, then a possible interpretation is that the source file above compiles because the call to std::launder produces a pointer to u.a.y. That interpretation is apparently not desirable.
[2020-11-21; Reflector prioritization]
Set priority to 3 during reflector discussions.
[2020-12-07; Davis Herring comments]
This issue is related to CWG 2464.
[2021-02-08; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4868.
Modify 17.6.5 [ptr.launder] as indicated:
template<class T> [[nodiscard]] constexpr T* launder(T* p) noexcept;[…]
-4- Remarks: An invocation of this function may be used in a core constant expressionwhenever theif and only if the (converted) value of its argument may be used ina core constant expressionplace of the function invocation. A byte of storage b is reachable through a pointer value that points to an object Y if there is an object Z, pointer-interconvertible with Y, such that b is within the storage occupied by Z, or the immediately-enclosing array object if Z is an array element. […]
Section: 24.7.11.3 [range.join.iterator] Status: Tentatively Ready Submitter: Michael Schellenberger Costa Opened: 2020-11-15 Last modified: 2020-11-21
Priority: 0
View other active issues in [range.join.iterator].
View all other issues in [range.join.iterator].
Discussion:
There seems to be a copy & paste error in the join_view::iterator::operator->() specification. According to 24.7.11.3 [range.join.iterator] p8 it is specified as:
constexpr iterator_t<Base> operator->() const requires has-arrow<iterator_t<Base>> && copyable<iterator_t<Base>>;-8- Effects: Equivalent to return inner_;
Now inner_ is of type iterator_t<range_reference_t<Base>>. So it is unclear how that should be converted to iterator_t<Base>, or why the constraints concern the outer iterator type iterator_t<Base>. On the other hand returning outer_ would not make any sense here.
As far as I can tell we should replace iterator_t<Base> with iterator_t<range_reference_t<Base>> so that the new specification would readconstexpr iterator_t<range_reference_t<Base>> operator->() const requires has-arrow<iterator_t<range_reference_t<Base>>> && copyable<iterator_t<range_reference_t<Base>>>;-8- Effects: Equivalent to return inner_;
Generally it would help readability of the specification a lot if we would introduce some exposition only aliases:
using OuterIter = iterator_t<Base>; //exposition-only using InnerIter = iterator_t<range_reference_t<Base>> //exposition-only
and use them throughout join_view::iterator.
[2020-11-21; Reflector prioritization]
Set priority to 0 and status to Tentatively Ready after six votes in favour during reflector discussions.
Proposed resolution:
This wording is relative to N4868.
Modify 24.7.11.3 [range.join.iterator], class template join_view::iterator synopsis, as indicated:
template<input_range V> requires view<V> && input_range<range_reference_t<V>> && (is_reference_v<range_reference_t<V>> || view<range_value_t<V>>) struct join_view<V>::iterator { private: using Parent = // exposition only conditional_t<Const, const join_view, join_view>; using Base = conditional_t<Const, const V, V>; // exposition only using OuterIter = iterator_t<Base>; //exposition-only using InnerIter = iterator_t<range_reference_t<Base>> //exposition-only static constexpr bool ref-is-glvalue = // exposition only is_reference_v<range_reference_t<Base>>; OuterIteriterator_t<Base>outer_ = OuterIteriterator_t<Base>(); // exposition only InnerIteriterator_t<range_reference_t<Base>>inner_ = // exposition only InnerIteriterator_t<range_reference_t<Base>>(); Parent* parent_ = nullptr; // exposition only constexpr void satisfy(); // exposition only public: […] iterator() = default; constexpr iterator(Parent& parent, OuterIteriterator_t<Base>outer); constexpr iterator(iterator<!Const> i) requires Const && convertible_to<iterator_t<V>, OuterIteriterator_t<Base>> && convertible_to<iterator_t<InnerRng>, InnerIteriterator_t<range_reference_t<Base>>>; constexpr decltype(auto) operator*() const { return *inner_; } constexpr InnerIteriterator_t<Base>operator->() const requires has-arrow<InnerIteriterator_t<Base>> && copyable<InnerIteriterator_t<Base>>; constexpr iterator& operator++(); […][…]
constexpr void satisfy(); // exposition only-5- Effects: Equivalent to:
[…] if constexpr (ref-is-glvalue) inner_ = InnerIteriterator_t<range_reference_t<Base>>();constexpr iterator(Parent& parent, OuterIteriterator_t<Base>outer);[…]
constexpr iterator(iterator<!Const> i) requires Const && convertible_to<iterator_t<V>, OuterIteriterator_t<Base>> && convertible_to<iterator_t<InnerRng>, InnerIteriterator_t<range_reference_t<Base>>>;[…]
constexpr InnerIteriterator_t<Base>operator->() const requires has-arrow<InnerIteriterator_t<Base>> && copyable<InnerIteriterator_t<Base>>;-8- Effects: Equivalent to return inner_;
Section: 24.7.16.3 [range.elements.iterator] Status: Tentatively Ready Submitter: Tim Song Opened: 2020-11-18 Last modified: 2021-02-08
Priority: 2
View other active issues in [range.elements.iterator].
View all other issues in [range.elements.iterator].
Discussion:
This compiles but the resulting view is full of dangling references:
std::vector<int> vec = {42};
auto r = vec | std::views::transform([](auto c) { return std::make_tuple(c, c); })
| std::views::keys;
This is because elements_view::iterator::operator* is specified as
constexpr decltype(auto) operator*() const { return get<N>(*current_); }
Here *current_ is a prvalue, and so the get<N> produces a reference into the materialized temporary that becomes dangling as soon as operator* returns.
We should either ban this case altogether, or make operator* (and operator[]) return by value when *current_ is a prvalue and the corresponding tuple element is not a reference (since this get is std::get, we need not worry about weird user-defined overloads.)[2020-11-29; Reflector prioritization]
Set priority to 2 during reflector discussions.
[2021-01-31 Tim adds PR]
[2021-02-08; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4878.
Modify 24.7.16.2 [range.elements.view], as indicated:
namespace std::ranges {
template<class T, size_t N>
concept has-tuple-element = // exposition only
requires(T t) {
typename tuple_size<T>::type;
requires N < tuple_size_v<T>;
typename tuple_element_t<N, T>;
{ get<N>(t) } -> convertible_to<const tuple_element_t<N, T>&>;
};
template<class T, size_t N>
concept returnable-element = is_reference_v<T> || move_constructible<tuple_element_t<N, T>>;
template<input_range V, size_t N>
requires view<V> && has-tuple-element<range_value_t<V>, N> &&
has-tuple-element<remove_reference_t<range_reference_t<V>>, N> &&
returnable-element<range_reference_t<V>, N>
class elements_view : public view_interface<elements_view<V, N>> {
[…]
};
}
Modify 24.7.16.3 [range.elements.iterator] as indicated:
namespace std::ranges { template<input_range V, size_t N> requires view<V> && has-tuple-element<range_value_t<V>, N> && has-tuple-element<remove_reference_t<range_reference_t<V>>, N> && returnable-element<range_reference_t<V>, N> template<bool Const> class elements_view<V, N>::iterator { // exposition only using Base = maybe-const<Const, V>; // exposition only iterator_t<Base> current_ = iterator_t<Base>(); // exposition only static constexpr decltype(auto) get-element(const iterator_t<Base>& i); // exposition only public: […] constexpr decltype(auto) operator*() const { returnget<N>get-element(*current_); } […] constexpr decltype(auto) operator[](difference_type n) const requires random_access_range<Base> { returnget<N>get-element(*(current_ + n)); } }; }static constexpr decltype(auto) get-element(const iterator_t<Base>& i); // exposition only-?- Effects: Equivalent to:
if constexpr (is_reference_v<range_reference_t<Base>>) { return get<N>(*i); } else { using E = remove_cv_t<tuple_element_t<N, range_reference_t<Base>>>; return static_cast<E>(get<N>(*i)); }
Modify 24.7.16.4 [range.elements.sentinel] as indicated:
namespace std::ranges { template<input_range V, size_t N> requires view<V> && has-tuple-element<range_value_t<V>, N> && has-tuple-element<remove_reference_t<range_reference_t<V>>, N> && returnable-element<range_reference_t<V>, N> template<bool Const> class elements_view<V, N>::sentinel { // exposition only […] }; }
Section: 24.7.12.3 [range.split.outer] Status: Tentatively Ready Submitter: Tim Song Opened: 2020-11-20 Last modified: 2021-02-08
Priority: 2
View all other issues in [range.split.outer].
Discussion:
Prior to the application of P1862R1, the part of split_view::outer-iterator::operator++ that searches for the pattern is specified as:
do {
const auto [b, p] = ranges::mismatch(current, end, pbegin, pend);
if (p == pend) {
current = b; // The pattern matched; skip it
break;
}
} while (++current != end);
P1862R1, trying to accommodate move-only iterators, respecified this as
do {
auto [b, p] = ranges::mismatch(std::move(current), end, pbegin, pend);
current = std::move(b);
if (p == pend) {
break; // The pattern matched; skip it
}
} while (++current != end);
but this is not correct, because if the pattern didn't match, it advances current to the point of first mismatch, skipping elements before that point. This is totally wrong if the pattern contains more than one element.
Consider std::views::split("xxyx"sv, "xy"sv):at the beginning, current points to the first 'x'
ranges::mismatch produces [iterator to second 'x', iterator to 'y' in pattern]
current now points to second 'x'
we increment current in the condition, so it now points to 'y'
At this point there's no way we can find the "xy" in the middle. In fact, in this particular example, we'll increment past the end of the source range at the end of the third iteration.
[2020-11-29; Reflector prioritization]
Set priority to 2 during reflector discussions.
[2021-02-08; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4868.
Modify 24.7.12.3 [range.split.outer] as indicated:
constexpr outer-iterator& operator++();-6- Effects: Equivalent to:
const auto end = ranges::end(parent_->base_); if (current == end) return *this; const auto [pbegin, pend] = subrange{parent_->pattern_}; if (pbegin == pend) ++current; else if constexpr (tiny-range<Pattern>) { current = ranges::find(std::move(current), end, *pbegin); if (current != end) { ++current; } } else { do { auto [b, p] = ranges::mismatch(std::move(current), end, pbegin, pend);current = std::move(b);if (p == pend) { current = b; break; // The pattern matched; skip it } } while (++current != end); } return *this;